Since the mid-1800s, ships have used bilge keels to mitigate roll motions due to waves. The use of bilge keels to minimize ship roll motion was first suggested by Froude. Historically, bilge keels have featured flat plate designs, and later also included discontinuous fin or wedge type designs along the ship's length. Conventional bilge keels are used to mitigate and dampen small to moderate roll motions.
A bilge keel is a projection like a fin extending from the fore to the rear part of the hull around the turn of the bilge on both sides of the ship. Bilge keels should be situated so that they do not strike the wharf or another vessel when tying alongside. Bilge keels are usually constructed from flat plates the form a sharp obstruction to the roll motion. The bilge keel itself should be aligned with the flow streamline of the moving ship so that its effect on the resistance is minimal.
The alignment can be determined by an oil dot flow technique during the initial resistance experiments. Visualization studies can be performed with tuffs placed on the hull surface during the resistance tests to aid and confirm the adequacy of the bilge keel alignment. The bilge keel alignment test is typically conducted at the ship design speed and a design craft displacement with the model tested in Froude scale.
A moving ship will induce a boundary layer along the hull surface. The boundary layer velocity profiles are sensitive to Reynolds scale. By testing a model in Froude scale, the Reynolds numbers can differ by two orders of magnitude between ship and model. The streamline orientation can differ noticeably between full-scale and model-scale due to large difference in Reynolds number. Bilge keels that are carefully aligned with streamlines during the model tests may experience an angle of attack on the full-scale vessel due to difference in boundary layer profiles along the ship hulls. The misalignment in flow field and bilge keel placement will induce added drag on bilge keels.
Similarly, craft displacement may change over the life of the vessel or even change during missions as fuel is consumed during the voyage. The distribution of weight on the vessel may also change. The change in displacement and weight distribution will change the craft sinkage and trim, which will shift the flow patterns around the hull and change the flow into the bilge keels. An added drag on bilge keels is induced by the misalignment of the flow direction with the bilge keel orientation. The same comments on Reynolds number effects are applicable to the effects of ship speed changes. In addition to the design speed, a ship is often operated at different ship speeds for different mission requirements. The change in ship speeds can change ship trim angle and flow streamline orientation, which can lead to an increase in the added drag on the bilge keels. It is desired to have a simple and effective bilge keel design with enhanced roll stabilization that alleviates added bilge keel drag, and reduces fuel cost due to changes in craft loading condition, ship speed, and Reynolds scale effects on boundary layer velocity profiles as described above, and generally accommodates for changing environmental conditions.